$ D = \left[\begin{array}{r}3 \\ 3 \\ 1\end{array}\right]$ What are the dimensions of matrix $ D$ ?
Answer: The first dimension is the number of rows in the matrix. How many rows does $ D$ have? The second dimension is the number of columns in the matrix. How many columns does $ D$ have? So, $ D$ is a 3 $\times$ 1 matrix.